"During the coding period, I will first work on the boot loader. I intend to modify the existing x86 boot loader so that it is capable of loading both a 32-bit Haiku kernel and a 64-bit one. Once this is done, I will work on implementing the x86_64 architecture functionality in the kernel. Finally, I will port modules and drivers to the 64-bit kernel. Should I have time, I will also begin work on porting userland." Heck. Yes.

The primary difference between Pentiums and newer vs 486 and older was the introduction of Virtual Interrupts. That simplified and sped up the interrupt handling for user mode code, making a significant difference in speed and coding for "enhanced" mode software on the x86. Intel would later work Virtual Ints into certain models of the 486 for people who wished the feature without needing to update the entire computer.

When you see software that runs on a Pentium/586 or better, and not on older hardware, that's usually the hardware that's being relied on that prevents it from being used on the older CPUs. Particularly with OSes, which need to handle both user and kernel modes.